As an automatic assembling line for automatically assembling a piston ring to a piston for an internal-combustion engine, there is known one that includes a frame, a guide rail provided to extend in a horizontal direction above the frame, a conveyer device that is guided along the guide rail, and others and that has a configuration where a plurality of pairs of holding arms for holding from a lower side a large-end portion of a connecting rod connected with a piston are aligned at predetermined intervals in order to horizontally convey the piston which is in a downwardly suspended state in this conveyer device (see, e.g., Patent Document 1 and Patent Document 2).
On the other hand, the connecting rod applied to the internal-combustion engine includes a small-end portion for connecting a piston, a large-end portion to be connected to a crankshaft, and a rod portion for coupling the small-end portion with the large-end portion, and the large-end portion is divided into two pieces, i.e., a rod-side component including the small-end portion and the rod portion and a cap component and configured to be fastened by a bolt.
Meanwhile, to convey a component assembly including the connecting rod and the piston to a bearing assembling station by using the conveyer device in the automatic assembling line and assemble a bearing for supporting the crankshaft to the large-end portion of the connecting rod, the cap component fastened by the bolt must be separated from the rod-side component in advance.
Therefore, a bolt utilized to fasten the cap component to the rod-side component is first loosened in a bolt loosening station on an upstream side, and then the cap component must be separated from the rod-side component together with the bolt in a cap component separating station on a downstream side.
However, in a state where the loosened bolt is inserted and held in an insertion hole of the cap component, when carrying the component assembly from the bolt loosening station on the upstream side to the cap component separating station on the downstream side, rescrewing that the once loosened bolt is unintentionally meshed with a female screw of the rod-side component (the bolt inserted and held in the cap component is again screwed into the female screw of the rod-side component) may occur due to vibration and others during conveyance.
When the rescrewing occurs in this manner, the cap component cannot be separated from the rod-side component in the cap component separating station on the downstream side, and the bearing cannot be assembled in the bearing assembling station placed on the downstream side.    Patent Document 1: Pamphlet of International Publication No. WO2005/000520A1    Patent Document 2: Pamphlet of International Publication No. WO2005/056999A1